The heterozygous deletion of a chromosomal region of human 22q11 is the genetic basis for several developmental defects with variable clinical severity. The clinical presentations may fit the diagnostic criteria for DiGeorge syndrome (DGS), Velocardiofacial syndrome (VCFS) or be very mild or complex. As the deletion is the unifying molecular element of these apparently diverse clinical entities, we will refer to these as the deletion 22q11 (del22q11) syndrome. Congenital heart disease, craniofacial anomalies, thymic and parathyroid aplasia and mental retardation are common findings in this syndrome. The 22q11 deletion, with an estimated incidence of 1:4000 live births, is one of the most frequent human chromosomal deletions associated with an abnormal phenotype. Almost twenty genes have been identified so far in the deleted region. However, a number of fundamental biological questions remains to be answered about the del22q11 syndrome: a) Is this a single or multiple gene disorder? b) Are any of the genes so far isolated from the deleted region relevant for the phenotype? c) What are the developmental pathways affected by the del22q11? To answer these questions, we propose to generate a panel of mouse embryonic stem (ES) cell lines carrying deletions, balancing duplications and single gene disruptions within the murine region homologous to the de122q11 region. These will be obtained using standard homologous recombination and a newly developed Cre-loxP strategy. Deletions will be designed to mimic the human mutations. The ES cell lines will be injected into mouse blastocysts to obtain the germ line transmission of the mutations. The phenotypic effects of deficiencies, single gene disruptions and combinations of these will be tested in vivo. This should allow us to perform a detailed genotype phenotype correlation study. Finally, the phenotype-generating deficiencies will be complemented in vivo to identify critical gene(s). This will be achieved using engineered balancer chromosomes or by reintroduction of genomic segments using standard mouse transgenic technologies. With this approach we will be able to dissect the del22q11 syndrome genetically and obtain mouse mutants which will be used to study the development of tissues and organs affected by this syndrome.